Well, as it turns out there's a movie in the works on this very scenario. I really hope they don't make it suck. My guess is it'll be geared towards the viewing pleasure of the audience and less towards what would actually happen, which means there will be a scene or three where the Marines try to hold a line and fire rifles into the full mass of Romans, and not much in the way of a couple of forward observers annihilating the Romans with an artillery strike or two. The latter is the tactic that would be chosen if this became a really real reality, to be followed up with a nightime raid by a few good Marines ricky-reconning into the bed chambers of the Roman Emperor and other leaders and killing them softly with Ka-Bars and suppressed rifles.